A nickel-based superalloy known to the art as Nimonic-75 consists of a class of materials which solidify from the molten state according to monovariant eutectic reactions, providing aligned polyphase structures including such systems as the ternary alloys identified as nickel-chromium-carbon and nickel-titanium-chromium-iron. The advantage of alloy compositions of this nature is that the desired microstructure can be achieved over a range of compositions within a given system. This provides a substantial increase in the freedom of selection of compositions, permitting increased optimization of properties.
It has been recognized in the art that directional solidification can enhance the mechanical properties of a particular alloy. Directional solidification involves the formation of a solid phase, e.g., chromium carbide fibers, during the transition from the molten phase. This solidification usually occurs in a particular axial direction. Continued cooling results in additional solidification in the same axial direction as the initial formation. The resulting solidified alloy is immensely strong in that axial direction. See, e.g., U.S. Pat. No. 4,111,723 to Lemkey et al.
The manipulation of alloy compositions to enhance certain properties is known to the art. Slight changes in composition can have a dramatic effect on mechanical strength and toughness. Certainly, the concept of directional solidification is based in part on identifying eutectic compositions wherein the chromium carbide fibers form in the molten phase of the alloy to provide a nucleus for further solidification.
An explanation why the Lemkey et al. composition obtains its attributes is found in a publication, "The Influence of Off-Axis Reinforcement on the Tensile Strength of an Ni-Al-Cr-C Eutectic Composite," Journal of Materials Science 10 (1975), 77-82. As there explained, the presence of aluminum permits the superalloy to form a nickel-aluminum-chromium-carbon eutectic composite, with a nickel-rich matrix containing a dispersion of Ni.sub.3 Al precipitate. Cr.sub.3 C.sub.2 fibers grow with their axes parallel to the crystallographic axis. As understood, the Cr.sub.3 C.sub.2 fibers and precipitates of Ni.sub.3 Al provide the cellular morphology specific to a Lemkey et al. composition. The author, Mr. May, concludes that the superior tensile strength of this composite requires both the presence of aluminum and of chromium carbide fibers.
The present invention provides superalloys having greatly improved mechanical properties. However, my superalloys are not dependent upon directional solidification to provide these enhanced properties, although over the range of compositions present in this invention, there are undoubtedly phases wherein eutectic formation occurs. Directional solidification is not critical to the desired properties, but is intended to fall within the scope of the appended claims, since the present invention achieves its mechanical properties without the presence of aluminum and, therefore, without the directional solidification technique of Lemkey et al.
The present invention comprises an improvement in the mechanical properties of a superalloy through the addition of minor amounts of rhenium and technetium and their mixtures, and optionally, erbium, thorium, uranium or another element of the lanthanide or actinide series. The addition of these materials provides a surprising and unexpected result which can be quantified, in part, by an increase in time to stress rupture at 800.degree. C. of several thousand hours. This unexpected increase permits the use of the improved superalloy in gas turbine engine component manufacture because of its enhanced resistance to failure under stress at high temperatures. Another surprising and unexpected result is that the order of magnitude increase in mechanical properties can be obtained without a corresponding order of magnitude increase in the cost of the improved superalloy.
It is, therefore, an object of this invention to provide an improved superalloy composition with enhanced mechanical properties.
It is another object of this invention to provide an improved superalloy composition at a price comparable to currently available superalloy compositions.